This invention relates to a circuit for generating a voltage waveform at an output node. The circuit is preferably included in an apparatus for use in processing charged particles, e.g. for use in performing mass spectrometry or ion mobility spectrometry. The present invention also relates to associated apparatuses and methods.
Numerous methods of performing mass spectrometry rely on the application of high frequency voltage waveforms to components of a mass spectrometer. For example, ion optical devices such as ion guides, ion traps, mass filters and other devices may all require such voltage waveforms, as is well described in the literature.
The quadrupole ion trap and quadrupole mass filter were first disclosed in U.S. Pat. No. 2,939,952 by W. Paul and H Steinwedel, in which several electrode structures to which radiofrequency waveforms can be applied are disclosed. The application of high frequency voltage waveforms to the electrode structures disclosed in this document produces a quadrupolar electric field which confines ions with certain characteristics to the device. Advancement of the theory has progressed and the state of the art has been well described in the book “Quadrupole Storage Mass Spectrometry” by R. E. March and R. J Hughes (Wiley Interscience) and the series of books edited by R. E. March and J. F. J. Todd, “Practical Aspects of Trapped Ion Mass Spectrometry”, Volumes I-V (CRC Press).
Commercially available means for generating voltage waveforms suitable for use in trapping or confining ions in mass spectrometry have tended to rely on a resonant circuit.
Resonant circuits produce essentially sinusoidal voltage waveforms such as that shown in FIG. 1. Resonant circuits are well known for generating sinusoidal radiofrequency (“RF”) voltage waveforms, and are used often in mass spectrometers or other devices. See, for example U.S. Pat. No. 2,939,952. The present inventors have observed that because such voltage waveforms are generated in a resonant manner, they are extremely efficient, needing little drive power to produce waveforms of high amplitude. However, it can be difficult to change the operating frequency of voltage waveforms produced by resonant circuits as to do this would usually require components making up the resonant circuit to be changed. Therefore in cases where it is desired to alter the operating frequency during operation, it is normally necessary to use a direct switching method that involves direct switching between different voltage levels.
Direct switching methods provide a significant advantage over resonant circuits in that the frequency of the voltage waveform can be easily changed during operation. However, existing direct switching techniques, which may be characterised as “hard switching” techniques, tend to be inefficient and power consumption is relatively large.
Examples of generating a high frequency waveform by direct switching methods that incorporate the use of a set of switches which are used to switch a voltage between two voltage levels by activating the switches alternately to generate a rectangular waveform are described in WO 01/29875 and U.S. Pat. No. 7,193,207B1, for example.
RU 2010392 is another example of a circuit for generating a voltage waveform at an output node, which uses a complex switching arrangement to switch between two voltage levels.
Circuits for generating voltage waveforms have been disclosed outside the field of mass spectrometry, see e.g. US2011/062935, U.S. Pat. No. 5,081,400, U.S. Pat. No. 7,026,765, KR20080042624, WO01/29875.
U.S. Pat. No. 7,755,034 discloses a quadrupole on trap and methods of dissociating ions held within the quadrupole on trap.